PGE2 has long been known as a potentiator of acute inflammation, but its mechanisms of action still remain to be defined. In this study, we employed inflammatory swelling induced in mice by arachidonate and PGE2 as models and dissected the role and mechanisms of action of each EP receptor at the molecular level. Arachidonate- or PGE2-induced vascular permeability was significantly reduced in EP3-deficient mice. Intriguingly, the PGE2-induced response was suppressed by histamine H1 antagonist treatment, histidine decarboxylase deficiency, and mast cell deficiency. The impaired PGE2-induced response in mast cell–deficient mice was rescued upon reconstitution with wild-type mast cells but not with EP3-deficient mast cells. Although the number of mast cells, protease activity, and histamine contents in ear tissues in EP3-deficient mice were comparable to those in wild-type mice, the histamine contents in ear tissues were attenuated upon PGE2 treatment in wild-type but not in EP3-deficient mice. Consistently, PGE2–EP3 signaling elicited histamine release in mouse peritoneal and bone marrow–derived mast cells, and it exerted degranulation and IL-6 production in a manner sensitive to pertussis toxin and a PI3K inhibitor and dependent on extracellular Ca2+ ions. These results demonstrate that PGE2 triggers mast cell activation via an EP3–Gi/o–Ca2+ influx/PI3K pathway, and this mechanism underlies PGE2-induced vascular permeability and consequent edema formation.

To understand physiological roles of tissue mast cells, we established a culture system where bone marrow-derived immature mast cells differentiate into the connective tissue-type mast cell (CTMC)-like cells through modifying the previous co-culture system with Swiss 3T3 fibroblasts. Our system was found to reproducibly mimic the differentiation of CTMCs on the basis of several criteria, such as granule maturation and sensitivity to cationic secretagogues. The gene expression profile obtained by the microarray analyses was found to reflect many aspects of the differentiation. Our system is thus helpful to gain deeper insights into terminal differentiation of CTMCs.

Membrane-associated guanylate kinase inverted 2 (MAGI-2) is a component of the slit diaphragm (SD) of glomerular podocytes. Here, we investigated the podocyte-specific function of MAGI-2 using newly generated podocyte-specific MAGI-2-knockout (MAGI-2-KO) mice. Compared with podocytes from wild-type mice, podocytes from MAGI-2-KO mice exhibited SD disruption, morphologic abnormalities of foot processes, and podocyte apoptosis leading to podocyte loss. These pathologic changes manifested as massive albuminuria by 8 weeks of age and glomerulosclerosis and significantly higher plasma creatinine levels at 12 weeks of age; all MAGI-2-KO mice died by 20 weeks of age. Loss of MAGI-2 in podocytes associated with decreased expression and nuclear translocation of dendrin, which is also a component of the SD complex. Dendrin translocates from the SD to the nucleus of injured podocytes, promoting apoptosis. Our coimmunoprecipitation and reconstitution studies showed that dendrin is phosphorylated by Fyn and dephosphorylated by PTP1B, and that Fyn-induced phosphorylation prevents Nedd4-2-mediated ubiquitination of dendrin. Under physiologic conditions, phosphorylated dendrin localized at the SDs; in the absence of MAGI-2, dephosphorylated dendrin accumulated in the nucleus. Furthermore, induction of experimental GN in rats led to the downregulation of MAGI-2 expression and the nuclear accumulation of dendrin in podocytes. In summary, MAGI-2 and Fyn protect dendrin from Nedd4-2-mediated ubiquitination and from nuclear translocation, thereby maintaining the physiologic homeostasis of podocytes, and the lack of MAGI-2 in podocytes results in FSGS.

Adipogenesis is a crucial aspect in controlling body fat mass ( 1, 2 ). Acquisition of the mature adipocyte phenotype is a highly regulated process in which mesenchymal stem cells (MSC) undergo differentiation, resulting in both an increase in size and number of mature adipocytes in adipose tissue. Adipose tissue is important not only for energy storage but also as an endocrine organ that regulates energy homeostasis by secreting various adipokines, such as cytokines, chemokines, growth factors, and lipid mediators ( 3 ). The presence of receptors for adipokines in preadipocytes and adipocytes has been shown, suggesting that secreted adipokines have autocrine effects and regulate their own differentiation and functions ( 4 ). Although it has been shown that a number of factors, including adipokines, regulate adipogenesis in various settings, most of the evidence comes from supraphysiological or pharmacological doses of these molecules to elicit a response. Hence, their physiological signifi cance in local milieu has not been established.Prostaglandins (PG) are arachidonate metabolites synthesized by the action of cyclooxygenase (COX) as the rate-limiting enzyme. COX has been shown to exist as two isomers, COX-1 and COX-2. PGs exert a wide range of actions through their binding to plasma membrane receptors ( 5, 6 ). For instance, PGF 2 ␣ exerts its actions via specifi c interactions with the prostanoid FP receptor, which activates phopholipase C, resulting in phophatidylinositol breakdown ( 7 ). In contrast, PGE 2 exerts action through its Abstract The prostaglandin (PG) receptors EP4 and FP have the potential to exert negative effects on adipogenesis, but the exact contribution of endogenous PG-driven receptor signaling to this process is not fully understood. In this study, we employed an adipocyte differentiation system from mouse embryonic fi broblasts (MEF) and compared the effects of each PG receptor-defi ciency on adipocyte differentiation.